A capacitive distance sensing system is known from DE 40 06 119 C2. The system consists of a sensor or oscillator, respectively, a sending and a receiving capacitance, a demodulator and a switching amplifier. The oscillator comprises a condenser which can be externally influenced. An active electrode, a ground electrode and a screening electrode that is arranged between these two electrodes are associated with this condenser. The active electrode and the screening electrode are at the same electrical potential due to the fact that the active electrode is driven in low impedance manner towards the shielding electrode by means of a driver stage. The active electrode and the screening electrode are connected by a line and are controlled by the same output of the control unit.
DE 32 21 223 A1 discloses a capacitive proximity initiator. This consists of a fully-insulated tubular housing having a sensor electrode arranged at the end thereof. A guard electrode and a screening electrode are associated with the sensor electrode. The screening and guard electrodes are distinguished by their cylindrical ring-like shape. The protection and screening electrodes are arranged in the interior of the housing such as to be mutually coaxial, whereby the guard electrode is located at a short distance from the sensor electrode which is arranged at the end. The screening electrode is set at a short spacing from the guard electrode. The guard electrode is operated as an ‘active guard’ and is connected via an impedance converter 42 to the sensor electrode 13 so that the same signal as is present on the sensor electrode is continuously applied to the guard electrode.
From CH 661 391 A5, there is known a device for a circuit arrangement for switching electrical devices in non contact making manner. In this arrangement, a pulse generator delivers pulses to an electrode emitting an electrical field via a signal output of a driver circuit. The driver and evaluation circuit is connected at its signal input to a further electrode to which a reference potential is applied at the same time. Both electrodes are overlapped by a common electrode to form a condenser which detects changes in the surrounding field and is carried capacitively by the transmitting electrode. If, however, in an arrangement of this type an electrode is wetted with respect to a reference potential, e.g. a surrounding housing, then the electrical field is capacitively short circuited and it is no longer possible to detect changes in the surrounding field.
DE 697 19 321 T2 also depicts a displacement current sensor wherein an electrical field is produced and received by receiving electrodes. It is not apparent as to how the coupling to the driver/evaluation unit is effected.
DE 2 239 359 A depicts a capacitive rain sensor wherein the transmitting electrode is screened with respect to the receiving electrode by a ground electrode. Consequently, the detection of a change resulting from the approach of an object is easily affected by other environmental influences. The presence of humidity leads to a change in amplitude.
U.S. Pat. No. 7,019,540 B2 depicts an electrostatic microphone in which a voltage divider is provided. Consequently, there are different signals at different parts of the active resonant circuit. It is not apparent as to how the coupling to the driver/evaluation unit is effected.